When transferring, try to do it all in one day.
Re: you using coral snad and live sand. If your current take is established, then your coral sand IS live sand. You do not need to add additional live sand or switch to new live sand. Just add more of the coral sand that you already have. Now, if you do not like the size of your current sand and want to go smaller, then switch out the sand. Use 1/2 to 3/4 non live sand of the szie you want and 1/2 to 1/4 live sand of the SAME size sand you put in there. If not the same size, then the smaller grain with eventually settle below the large grain of sand.
Rock work: There should not be a pH spike from adding live rock, reef bones, or base rock....ESPECIALLY if your alkalinity is up. This will keep your pH stable. If you use live rock, make sure it is fully cured. Many LFS sell cured live rock, but it still have decompossing organisms on it. Put in a trash can of saltwater with a powerhead and soak it for a week or two. Cover it up with a lid of some sort so no or little light can get in there. Set it outside or in a garage as it will stink. As for reef bones and base rock, rinse off the particles before adding to the tank. If the reef bones seem fresh, treat them like uncured live rock and soak them. Reef Bones is a product by CaribSea that is sold as dead live rock and is dry. Sometimes I see is still with color and as a damp feeling to it. IF this be the case, cure it. If in doubt, cure it.
What I did with mine is that I set u the 75 on its stand and did all the plumbing work, but did not glue everything. After I found the way I wanted everything to be, I broke that down and set it aside. I had two large trash cans for my aquarium use only, a dozen buckets, and other containers that are only used for my aquarium. I filled up the buckets up about half way with aquarium water from the existing tank. I placed my corals in these buckets and made sure none touched. I save one bucket for my fish and I covered the bucket. Down here, it is quite warm, so my water temperature stayed consisted 78F. If you live further north, I suggest a couple of ice chest instead of buckets, and run a heater to make sure the water temperature doesn't drop too low.
Next I placed all my live rock in the trash cans and filled them up with aquarium water. I then emptied the substrate out into its own container. Removed my 55gal and set aside. Set up the 75gal and glued all the plumbing work. I shimmed and leveled the tank. Then, I added the substrate from the old tank. In my case, I actually broke down two tanks. I had a 20gal with the same size substrate that I have inverts in. I decided to break that on down and get rid of the tank. It gave me plenty of sunstrate for the 75gal. Next, I added my larger pieces of rock to form my base. I placed a few rocks in one corner where I set up a pump to start pumping the old water into the 75. I used the rock to let the ater trickle down so it didn't stir up the substrate and make the water really cloundy. I continued stacking rock as the water was being into the tank. I began to add my corals and the water they were in into the tank. I had extra water on hand that I pumped into the system. Lastly, I added my fish, ran my pumps and adjusted ball valves to get the flow I wanted.
I added additional rock to the tank and had no such pH spike. I did add a product call BioSpira which is concentrate bacteria just incase there was an ammonia spike. I also added BioSafe/BioCoat to help the fish fight stress. So far, the fish are doing fine, eating well, and show no signs of stress. Its been about 2 weeks or so since the change.
The corals will stress. No avoiding that. Being in a new lighting scheme and environment, their colors will not look as good as they should for a while. It can take up to a month for corals to adjust...especially hard corals, so be patient. My softie rebounded quickly. The LPS took a few days. My monties were the same. 2 of my 3 acroporas are still offcolor, by the polyps are openning and it is just a matter of time. One of my bird's nest did fair very well, but I simply broke off the dead branches to give room for the living branches.
I went from a 55gal tank with 440 watts of VHO to a 75gal with 550 watts of VHO.
Probably the most important thing is to use as much of the old water, substrate, and rock work as possible. This will have all your bacteria to help break down waste products. Going from a 55 to 75 was not a big deal the extra water was equivlent to a water change. Going from a 55 to a 180 is a big difference and once should add concentrated bacteral such as BioSpira to make sure the tank doesn't go through a cycle.
Hope some of this was helpful.
